Highly efficient light-emitting diodes based on fluorene copolymer consisting of triarylamine units in the main chain and oxadiazole pendent groups

Abstract

We have synthesized a fluorene-based copolymer, PFA-OXD, which possesses bipolar charge-transporting functionalities, through the incorporation of electron-rich triphenylamine moieties and electron-deficient oxadiazole pendent groups into the polymer backbone and onto the C9 position of each fluorene repeating unit, respectively. The propeller-shaped triphenylamine units in the main chain and the rigid cardo-type OXD side chains endow this polymer with a very high glass transition temperature (306 degrees C) and good thermal stability, without sacrificing its good solution processability. Both in dilute solution and as a thin film, PFA-OXD exhibits a bright blue emission upon excitation of either the main chain or oxadiazole side chains; this phenomenon indicates that efficient energy transfer occurs from the photoexcited pendent groups to the polymer backbone. An organic light-emitting device incorporating this copolymer as the emitting layer exhibits a voltage-independent and stable blue emission having color coordinates (0.14, 0.15) at 9 V; the maximum brightness was 7128 cd m(-2) and the maximum luminance efficiency was 2.07 cd A(-1). In addition, doping PFA-OXD as a host material with 2.6 wt % of a red-emitting phosphorescent dye, Os(fppz), led to a high-brightness, highly efficient red electroluminescent device having CIE coordinates of (0.65, 0.34), a maximum brightness of 28 440 cd m(-2), and a maximum external efficiency of 9.30%.